I have a 79 300D, NA that does not have a factory installed block heater.
Everyone I spoke with said if its not there now, dont bother trying to get it in, as you can crack the block trying to get the plug out.
I also had the old coil style glow plugs, and an old battery.
It get pretty cold here up in New Hampshire. I replaced the battery with a 900CCA battery, I put new glow plugs in with the plug to plug circuit wiring, (I left the existing relay, so it still glows for 20-30 seconds before the dash light goes out). I did purchase the lower radiator hose heater from dieselgiant, and I also purchased two pad heaters. One is affixed to my oil pan, and the other is a pad heater that goes underneath your battery. I installed all this a few weeks ago, so we'll see what happens when the temps drop. I figure between the warm coolant in the block, warm oil in the pan, and a warm battery I should be all set.
I'm going to have to run all these on a timer, or my electric bill will be sky high !!
Good Luck.
http://www.padheaters.com/description.html